1. Field of the Invention
The present invention relates to a lubricant. In particular, the present invention relates to a lubricant which has a stable lubrication property for a long time.
2. Description of the Related Art
Since a fluorine based lubricant has in general excellent lubrication properties while it is expensive demands for fluorine base lubricants are increasing in various fields, particularly in fields which require a high lubrication property, though the used amount of lubricant is small.
The magnetic recording media is one typical example of such a field. In particular, the fluorine based lubricant is indispensable for the ferromagnetic metal thin film magnetic recording medium which has excellent high density recording properties, because its magnetization resistance is easily increased and its thickness easily decreased, while its surface smoothness is high, no binder resin having toughness being used, so the coefficient of friction against a magnetic head is large. Thus, the thin film medium is easily abraded or damaged in comparison with a coating type magnetic recording medium which is prepared by coating a magnetic paint comprising a magnetic powder, a binder resin, an organic solvent and other necessary components on a nonmagnetic support and drying the paint.
For example, the durability and traveling property are improved by using a perfluoropolyether lubricant or a partially fluorinated carboxylic acid ester lubricant by adding it in a magnetic layer or applying it on the ferromagnetic metal thin film (cf. Japanese Patent KOKAI Publications No. 236118/1987 and 210615/1990).
However, by the addition of the conventional perfluoropolyether lubricant or partially fluorinated carboxylic acid ester to the magnetic layer, the durability is not sufficiently increased. When such a lubricant is applied on the ferromagnetic metal thin film, the lubricating effect is not sufficient and adequate reliability is not attained under severe conditions, such as a static picture, namely a still state in which the same part of the medium is continuously contacted to a magnetic head, since the holding force of the lubricant on the ferromagnetic metal thin film is low even though it has an excellent lubrication effect.
It is proposed to introduce a polar functional group, such as a hydroxyl group, or an ester group at a molecular end of the perfluoropolyether so that the lubricant is firmly chemically absorbed on the surface of the ferromagnetic metal thin film (cf. Japanese Patent Publication No. 10368/1985). However, such a perfluoropolyether lubricant is dissolvable only in an expensive solvent, such as FREON (a trademark) or a perfluoroalkane, and its cost is uneconomical. Further, since hydrofluoric acid is generated by pyrolysis during the recovery of a such lubricant, the working safety deteriorates. In addition, the use of a solvent containing a large amount of halogens is undesirable in view of the protection of the global environment.
In recent years, a fluorine based lubricant which can be dissolved in a hydrocarbon solvent instead of the special fluorine base solvent has been developed. For example, a compound comprising a long chain hydrocarbon group bonded to a molecular terminal of a perfluoropolyether through an amide bond or an ester bond (cf. Japanese Patent KOKAI Publication Nos. 78016/1990 and 49128/1990).
However, in the above compound comprising a long chain hydrocarbon group bonded to a molecular terminal of a perfluoropolyether through an amide bond or an ester bond, the amide or ester bond is easily hydrolyzed on a metal surface having high chemical activity, and an acid, which is generated by the hydrolysis, corrodes the metal surface or forms a viscous salt. Therefore, the lubrication property is deteriorates.